This invention relates in general to methods and systems of rapid focusing and zooming for the applications in the projection of volumetric 3D images and in the imaging of 3D objects.
One category of V3D display generates V3D images by rapidly moving a screen to repeatedly sweep a volume and projecting 2D images on the screen. V3D images thus form in the swept volume by after-image effect. One typical mode of motion is to place a screen on a slider-crank mechanism to make the screen move in reciprocation motion. Tsao U.S. Pat. No. 6,765,566 (FIG. 20) describes a system with a screen that reciprocates by a rotary motion, as illustrated in FIG. 1. In principle, this is to revolve the screen 2031 about an axis 2000 and sweep a volume 2040 while keeping the screen surface always facing a fixed direction. For convenience, this is called “Rotary Reciprocating mechanism”. The advantage of Rotary Reciprocating mechanism is that a motion equivalent to reciprocation can be generated by smooth rotary motion without the need of linear bearings, which in general have higher cost, higher noise and shorter life than rotary bearings.
Another category of V3D display applies a stack of electrically switchable screens (usually liquid crystal materials) as display means. By quickly and sequentially switching different screens in the stack, a moving screen can be generated. 2D image frames can be projected onto the liquid crystal screen to create V3D images. For example, Sullivan (U.S. Pat. No. 6,100,862, which is incorporated herein for this invention by reference) describes such a system.
In these approaches, one major issue is to project images to the screen (or the liquid crystal screen) and keep the image in focus.
Paek (“A 3D Projection Display using PDLCs”, presented at the Conference of the International Society for Optical Engineering, January 29-February 2, San Jose Calif.) uses a piezoelectric material based simple lens for rapid focusing. However, practical projection lens requires multiple lens with certain size in order to provide bright and high quality image. To achieve this by piezoelectric lens can be difficult and costly. By similar reasoning, a vari-focal liquid lens, which can change its focal length by changing the voltage applied over its “electro-wetting” water-oil interface, can also be used. Vari-focal liquid lens is now commercially available (Varioptic of France, see www.varioptic.com). However, for the purpose of projecting bright, high quality image, liquid lens appears to be too small (only 2˜3 mm aperture) to pass enough light. And because its principle is based on surface tension between two liquids, it is doubtful that larger lens can be made. Liquid weight becomes more significant for larger lens.
Sullivan (U.S. Pat. No. 6,100,862) uses a varifocal mirror before a projection lens to adjust the focus of projection.
Sullivan (US Pat. Pub. No. 2003/0067421, which is incorporated herein for this invention by reference) describes a vari-focusing projection system. A rotating transparent disk with an azimuthally varying thickness is placed between the projection lens and the image source to change the effective object distance in a fast and periodical fashion. This is based on the principle that a transparent material (of refractive index>1) between lens and object changes (shortens) the effective object distance due to refraction. When effective object distance is shortened, image distance is increased.
However, it is difficult to apply Sullivan's disks to systems with reciprocating screen or “Rotary Reciprocating” screen (of FIG. 1). Sullivan describes basically three types of disk. FIG. 2a illustrates one type that has a thickness varying from the minimum at 0 degree to the maximum at 360 degree. FIG. 2b illustrates another type that has a thickness increasing from the minimum at 0 degree to the maximum at 180 degree and then decreasing back to the minimum at 360 degree. A Rotary Reciprocating screen of FIG. 1 moves back and forth in space. However, the disk of FIG. 2a has a discontinuous shape function with a jump at 201. Therefore, this type of disk can not be used in both directions of the screen motion. This is not desirable. The disk of FIG. 2b has a non-smooth shape function. Near maximum thickness 202, the slope of the disk surface makes a sudden change from 203 to 204. This happens near minimum thickness 205 too. Optically, this sudden slope change deflects the angle of optical path. Therefore, the optical path should not cover both sides of the maximum line 202 (or minimum line 205) at the same time. As a result, a significant portion of the display volume 2040 near top and bottom can not be used to display image. But more importantly, the function of thickness change of the disk is a linear function relative to rotating angle. It does not match the motion function of a screen in slider-crank motion or in Rotary Reciprocating motion (sinusoidal motion) (see Appendix E for the motion functions). Although it is possible to use a combination of rotary motion with a rack and pinion system to turn the disk rotation into a sinusoidal function relative to time, the resulted system can be noisy and complicated. Therefore, the disk of FIG. 2b is also undesirable. The third type of disk is a disk with stair-like surface (FIG. 5A, 5B of Sullivan). Because a stair step must be wide enough to allow projection beam to pass through, a disk of reasonable size can only contain a very limited number of stair steps. As a result, this type of disk is only suitable for displaying a very limited number of frames in the display volume.
In the field of imaging, Fantone et al. (U.S. Pat. No. 6,066,857) (FIG. 22-24) describes a similar vari-focusing imaging system used in a barcode reader. The system uses a rotating transparent disk with a helical surface on one side and a flat surface on the other side. A stationary wedge prism is placed near the flat side to form a small air gap. The rotating disk creates thickness variation. This disk also has a discontinuous shape and is not suitable for a screen that moves back and forth.
Tsao (U.S. Pat. No. 5,954,414) describes several image delivery systems that maintain not only focus but also constant magnification of projected image frames. One system is a moving reflector-pair placed between the projection lens and the moving screen. The moving reflectors compensate the change of optical path length caused by the motion of the screen. Tsao (U.S. Pat. No. 6,302,542) describes another image delivery system comprising a single moving flat reflector (see column 5, lines 1-5, 28-31, 37-39 of the referred patent). The reflector moves by a “Rotary Reciprocating mechanism” (similar to FIG. 1) in synchronization with the screen. However, these systems require certain amount of mechanical parts and take up certain amount of space.
Tsao U.S. Pat. No. 5,954,414 (column 7, line 47 to column 8 line 7) and U.S. Pat. No. 6,302,542 (column 5 line 55 to column 6 line 19) also describe a moving zoom lens system, which keeps the projected image in focus and maintains constant magnification. In general, a zoom lens can be separated into two lens groups. Zooming is achieved by moving the two lens groups separately but simultaneously. One method is using linear stages driven by a microcomputer-controlled servomotor or using cams to adjust the positions of lens groups. Another way to drive the stages is to use piezoelectric actuators. Another method is using lens with adjustable power. U.S. Pat. No. 6,302,542 (column 8, lines 24-55) also describes a “synchronized-focusing projector”, which achieves rapid focusing by adjusting lens position or power rapidly. The method also includes changing optical path length by moving a reflector, instead of moving the lens (U.S. Pat. No. 6,302,542, FIG. 6, column 6 line 5-19). The cost of linear stages, control systems and piezoelectric actuators can be significant. A cam driving system can also be complicated to design and can be noisy. Therefore, it is desirable to improve on these approaches.
A rapid focusing system can also be useful in the field of 3D shape imaging and recovery (or sometimes called volumetric measurement). In this field, one category of approach is based on the focus or defocus of multiple 2D images (pictures) of a 3D shape or a 3D scene. In the method of Shape from Focus (or Depth from Focus) (SFF or DFF), multiple 2D images of a 3D surface are taken at different focal depths. Image processing of the 2D images obtains a set of “focus measures” at each image point. The depth of a surface point is then obtained by finding the peak of the focus measure function by Gaussian interpolation of the focus measures. In the method of Depth from Defocus (DFD), depth information is computed by a “defocus function” from the blurred images of areas that are out of focus. The DFD method requires much fewer 2D images. Details of the SFF methods can be found in the following documents:    1. Nayar and Nakagawa, “Shape from Focus”, IEEE Transaction on Pattern Analysis and Machine Intelligence, vol. 16, No. 8, August 1994, pp. 824.    2. Noguchi and Nayar, “Microscopic shape from focusing using active illumination”, Proc. IEEE Conference on Computer Vision and Image Processing 1994, pp. 147-152.    3. Yun and Choi, “Fast Shape from Focus using Dynamic Programming”, in Three-Dimensional Image Capture and Applications III, Ed. by B. D. Corner and J. H. Nurre, Proceedings of SPIE Vol. 3958 (2000) pp. 71.Details of the DFD Methods can be Found in the Following Documents:    4. Watanabe et al., “Real-time computation of depth from defocus”, Proc. of SPIE v 2599, 1996, pp. 14-25.    5. Wallack U.S. Pat. No. 6,483,950, “Determining a depth”.The above documents are incorporated herein by reference.
In order to image a 3D object at different focal depths, one can displace the image sensor with respect to the image plane, or move the lens, or move the object with respect to the object plane. In the paper of Nayar and Nakagawa, the 3D object is placed on and moved by a movable stage. The depth map is computed from 10-15 images. In the paper of Yun and Choi, the camera was moved by a motorized or piezoelectric-actuated stage. In Watanabe et al.'s paper, two cameras positioned at different depths were used. The depth map is computed from only 2 image frames. When the range of focal depth change is small, a rotating sector wheel with glass plates of different respective index of refraction is placed before the 3D object to change effective focal distance. This is described in Wallack U.S. Pat. No. 6,483,950. In general, these methods of depth scanning are either slow or unable to cover large depth.
Accordingly, the purpose of this invention is to develop a rapid focusing system that has a simple structure and occupies small space, especially for V3D displays based on reciprocating screen or Rotary Reciprocating screen. This invention is also to develop a focusing system that does not have the shortcomings of the aforementioned rotating disks, that is, discontinuous or non-smooth shape. The purpose of this invention is also to develop a focusing system that can be driven by simple mechanisms and can be easily synchronized with the motion of the moving screen. Further, the purpose of this invention is also to have a system of reasonable cost. This includes using parts that are easy to manufacture and using simple mechanical elements of low cost and high reliability.
A rapid focusing system can be used in a camera system to rapidly scan the image plane or to scan the object plane without moving the camera body. The scanning range can cover large depth and the scanning speed can allow real-time 3D motion capture.